Warming blankets with channels are well known in the art and are available from a variety of sources. Many of these blankets are formed by weaving two layers of cloth simultaneously thereby creating a blanket with a pattern of channels, in which are disposed a single unitary elongate element which incorporates both heating and sensing functions. In the construction of warming blankets it is well known to use wrapped wire constructions in which complementary heating and sensing wires are wrapped around a structural core such as an elongate polymeric fiber or the like. In prior known constructions of this type the heating and sensor wires are disposed within a common insulated covering forming a unitary elongate structure which is then threaded in a desired pattern through channels at the interior of the blanket. The wires may be wrapped concentrically with an insulating sleeve between the wires such as disclosed in U.S. Pat. No. 6,153,856 or in a co-axial arrangement such as disclosed in U.S. Pat. No. 5,861,610 to Weiss. It is also known to use double wrapped wires with either a meltdown layer or temperature coefficient material between the two wires such as described in U.S. Pat. No. 4,742,212 to Ishii. In all of these constructions the heating wire and the sensor wire are ultimately disposed within a common elongate structure surrounded by a common outer insulating sleeve.
In operation of the dual wire constructions, an electrical current is passed through the heating and sensor wires causing the heating wire to increase in temperature. The electrical properties of the sensor wire change with temperature in a predetermined manner. Thus, by monitoring the applied current and voltage across the sensor wire, the temperature of the sensor wire can be determined. Based on the temperature of the sensor wire, the current to the heating wire can then be increased or decreased so as to raise or lower the temperature of the blanket as desired. While such dual wire constructions provide a degree of temperature control under steady state conditions, it has been found that it is difficult to hold the blanket at a substantially steady temperature when the room temperature undergoes a dramatic change. The current applicants hypothesize that such difficulty is due to the overpowering influence of the heating element on the sensor wire housed within the common sleeve structure.
It has been proposed to use a single wire wrapped around a textile core and covered by an insulating sleeve to carry out both the heating and sensing functions. For example, U.S. Pat. No. 6,222,162 to Keane discloses a copper cadmium alloy wire wrapped around a textile core and insulated to form an elongate structure. The insulated structure is channeled into a blanket shell and used for both heating and sensing. It has been found that such single wire constructions may give rise to difficulties in temperature regulation leading to the undesirable possibility of overheating.
It has also been proposed to utilize separate heating and sensing elements arranged in different patterns within the blanket. By way of example, such techniques are disclosed in U.S. Pat. No. 6,768,086 to Sullivan et al., the contents of which are incorporated herein by reference. While such practices may provide the benefit of measuring temperature over an extended area, incorporating the advocated multiple wiring patterns may give rise to an undesirable level of complexity. In particular, the use of distinct complex patterns for the heating and sensor wires may make it difficult to insert and maintain the wires in the desired orientation.